1. Field of the Invention
The present invention relates to a gate driving circuit for a power semiconductor switch, and more particularly to a pulse driving circuit for driving abruptly or steeply a power semiconductor switch into a conductive state at a very high speed.
2. Description of the Related Art
In the power energy field, there has been developed a pulse power technique. In the pulse power technique, energy is first stored at a very slow rate, and then the stored energy is instantaneously discharged within an extremely short time period such as 100 nano-seconds to obtain extremely high power not less than 10.sup.8 Watts. In order to discharge a large amount of energy as fast as possible, it is required to provide a high speed switching element which can operate at a high voltage. To this end, there have been proposed to utilize gap switch and thyratron. However, these switches could not be operated at a sufficiently high frequency and a life time of these switches is rather short.
Recently there have been developed various kinds of power semiconductor switching elements such as thyristor, static induction thyristor, gate turn-off thyristor (GTO) and insulated-gate bipolar transistor (IGBT), which have a high speed switching property at a rather high voltage and a relatively large current. These power semiconductor switching elements have been practically used. Particularly, a series circuit of these power semiconductor switching elements could be utilized as a high voltage power semiconductor switch for the pulse power application.
In order to drive the above mentioned power semiconductor switch in a pulse mode, the switch has to be turn-on as fast as possible. To this end, upon triggering, a large current raising very sharply must flow into a gate of the power semiconductor switch for an initial short time period of several tens nano-seconds (ns), and then an on-current of about 1 A has to be flown continuously for about 50 micro-seconds (.mu.s). There have been proposed several gate driving circuits which can drive the power semiconductor switch in the pulse mode just explained above.
FIG. 1 is an example of known gate driving circuits for driving a power semiconductor switch in a pulse mode. A first DC voltage source 1 for turning-on a semiconductor switch and a second DC voltage source 2 for turning-off a semiconductor switch are provided. A series circuit of a resistor 3 and a capacitor 4 is connected across the turn-on DC voltage source 1, and a junction point between the resistor 3 and the capacitor 4 is coupled with a gate G of a power semiconductor switch 6 by means of a turn-on switching element 5. A stray inductance contained in a circuit portion from the DC voltage source to the gate G of the power semiconductor switch 6 is represented as an inductor 7 which is connected between the turn-on switching element 5 and the gate G of the power semiconductor switch 6.
The above mentioned resistor 3 serves not only as a charging resistor for the capacitor 4 but also as a resistor for supplying a current to the gate G of the power semiconductor switch 6 for maintaining the power semiconductor switch in the conducting state. A cathode K of the power semiconductor switch 6 is connected to a negative terminal of the turn-on DC voltage source 1, and a junction point between the turn-on switching element 5 and the inductor 7 is coupled with a negative terminal of the turn-off DC voltage source 2 by means of a turn-off switching element 8. The turn-on and turn-off switching elements 5 and 8 are controlled by a control circuit 9.
Now the operation of the known gate driving circuit illustrated in FIG. 1 will be explained with reference to signal waveforms shown in FIG. 2. When the power semiconductor switch 6 is in the non-conducting state, the turn-on switching element 5 (SW5) is made off and the turn-off switching element 8 (SW8) is made on under the control of the control circuit 9. Therefore, the capacitor 4 is charged by the turn-on DC voltage source 1 by means of the resistor 3 to a voltage E.sub.1 which is equal to the output voltage of the turn-on DC voltage source 1.
At a time instant t.sub.0, the turn-on switching element 5 is switched from "off" to "on", and then energy stored in the capacitor 4 flows through the turn-on switching element 5 to the gate G of the power semiconductor switch 6, and further flows to the cathode K of the semiconductor switch. A maximum value of the current flowing from the gate G to the cathode K of the power semiconductor switch 6 is denoted as I.sub.2 in FIG. 2. Since the large current I.sub.2 flows to the gate G of the power semiconductor switch 6, this switch is turned-on and a large current flows through the anode-cathode A-K path by means of main DC voltage supply source not shown. After that, the power semiconductor switch 6 is kept conductive as long as the current flows into the gate G of the power semiconductor switch. At an instant t.sub.1, the turn-on switching element 5 is turned-off and the turn-off switching element 8 is turned-on by the control circuit 9, and then the power semiconductor switch 6 is turned-off.
A raising rate (di.sub.G /dt) of the gate current I.sub.G flowing into the gate G of the power semiconductor switch 6 when the turn-on switching element 5 is made on, is determined by the voltage E.sub.1 of the turn-on DC voltage source 1 and the stray inductance L.sub.S denoted by the inductor 7 in FIG. 1. That is to say, the raising rate (di.sub.G /dt) can be expressed by di.sub.G /dt=E.sub.1 /L.sub.S. Usually the stray inductance L.sub.S is about 100 nH and the raising rate (di.sub.G /dt) of the gate current is required not less than 3000 A/.mu.s. Therefore, the voltage E.sub.1 of the turn-on DC voltage source 1 has to be not lower than 300 V.
An amount of charge Q to be supplied to the gate G of the power semiconductor switch 6 for turning-on the power semiconductor switch at a high speed is determined by respective switches. This amount of charge Q is identical with an amount of charge stored in the capacitor 4, and its energy is represented by 1/2.times.QE.sub.1. An energy loss in the resistor 3 for storing such energy is also expressed by 1/2.times.QE.sub.1. Therefore, the turn-on DC voltage source 1 has to supply a sum of these energy and is equal to QE.sub.1.
Now it is assumed that the capacitor 4 has a capacitance of 0.5 .mu.F and a pulse repetition frequency is 2 KHz. Then the turn-on DC voltage source 1 must supply a power of 90 W. Since the turn-on DC voltage source 1 must supply the current for keeping the power semiconductor switch 6 conductive for 50 .mu.s, and this current amounts to a power of 30 W. Therefore, the turn-on voltage source 1 must supply a sum of these powers which amounts to a very large value of 120 W.